As worldwide telecommunications usage continues to expand, the need for more efficient and accurate means of processing and transporting data has become apparent. To satisfy the ever-increasing demand for greater bandwidth, research efforts over the last several decades have been directed toward developing optical signaling methodologies providing improved efficiency, increased accuracy and greater signal capacity. As a result of the incorporation of optical components into conventional telecommunication systems, substantial gains in channeling capacity, signal accuracy and transport efficiency have been realized.
For example, transmission of information by the propagation of optical signals in optical fibers is now widely used to increase the signaling rates in long haul-haul telecommunication systems. In these systems, optical signals are generated from electronic signals, transported along great distances via optical fibers and detected in a manner to regenerate the original electronic signal. Use of optical fiber for signal transmission is capable of achieving very high signal transmission rates ranging from several mega-bits per second to several tens of giga-bits per second. In addition, use of optical means for signal transmission has been shown to provide decreased propagation loss, higher channeling capacity and resistance to electromagnetic interference. As a result of these well-known advantages, optical communication components are pervasive in nearly all modern telecommunication networks.
To realize the full benefits provided by optical signal transmission and processing, substantial research has been directed toward the goal of developing purely optical telecommunication systems. In a purely optical telecommunication system, all aspects of signal generation, transmission, and processing are performed by optical methods and devices. While significant improvements in signal generation and transmission have been realized using optical methods, signal processing by purely optical methods remains a primary barrier to achieving the full benefits of optical telecommunications. To achieve the maximum efficiency and accuracy gains afforded by purely optical telecommunication systems, methods of direct optical processing without conversion to electronic signals are needed.
Research directed toward developing purely optical telecommunication systems has focused on a variety of optical signal processing applications that avoid the conversion of optical signals to electronic signals. Such applications require optical devices capable of performing the full spectrum of signal processing functions, such as filtering, amplification, beam splitting, switching, signal equalizing, signal coupling, wavelength multiplexing and wavelength demultiplexing. Low loss Fabry-Perot (FP) optical filters and interferometers, particularly fiber Fabry-Perot filters and interferometers, are important optical components in a great number of such optical devices.
A Fabry-Perot interferometer (FPI) consists of an optical resonance cavity formed between two reflectors, commonly substantially parallel reflectors. Typically, the reflector pair comprise partially transmitting, low loss reflectors. The optical path length through the resonance cavity may be selectably adjustable, thereby providing an interferometer having tunable transmission properties. Alternatively, the optical path length through the resonance cavity may be fixed, thereby providing an interferometer having fixed transmission properties. The basic structure and operation of an FP interferometer is well-known in the art and is described by Moore et al. in “Building Scientific Apparatus”, Addison-Wesley Publishing Co, 1989, pgs. 242–251.
FP interferometers pass narrow bands of light, the transmission bands of the filter, having center wavelengths, which satisfy the resonance condition of the interferometer. Specifically, when the optical path length of the round-trip length of the cavity is an integer of a wavelength, then that wavelength together with a narrow band resonates inside the cavity, and passes through the filter with very low losses. For a fixed FP cavity length, the resonant wavelength changes periodically. The period of the resonant wavelength is called free spectral range (FSR) of the filter and is provided by the equation:
                    FSR        =                  c                      2            ⁢            L                                              (        I        )            where c is the speed of light and L is the optical thickness of the resonance cavity. In a tunable FP etalon, the FSR and the resonant wavelengths are selectably adjustable by changing the optical path length through the resonance cavity.
Three types of FP tunable filters are typically used in fiber-optic communication systems: (1) lensed FP interferometers, (2) microelectromechanical system based FP filter (MEMS-FP filter), and (3) all-fiber FP interferometer. All-fiber FP interferometers are preferred for many telecommunications applications due to their exceptional stability, low cost and high optical throughput. The fabrication and use of fixed frequency and tunable FFP filters are described in U.S. patents including U.S. Pat. Nos. 5,892,582; 6,115,122; 6,327,036; 6,449,047; 6,137,812; 5,425,039; 5,838,437; and U.S. patent application Ser. Nos. 09/633,362; 09/505,083; 09/669,488, which are hereby incorporated by reference to the extent that they are not inconsistent with the disclosure in this application
FIG. 1 illustrates a typical fiber Fabry-Perot tunable filter (FFP-TF) formed in a ferrule assembly consisting of two reflectors 10 and 12 deposited directly onto fiber ends 9 and 11, respectively and a single-mode fiber (SMF) waveguide 20, (5 μm to 10 mm in length, held within a ferrule wafer) of selected length bonded to one mirror 10 (the embedded mirror). The internal end of the wafer 13 and the mirror-ended fiber end 11 are spaced apart to form an air-gap 21 within the cavity, the length of which can be selectively adjusted to tune the transmission properties filter. The FFP configuration of FIG. 1 (not drawn to scale) is illustrated as a fiber ferrule assembly in which fibers 5 and 7 each having a fiber core 22 and fiber cladding 23 are fixed within the axial bores of ferrules 1 and 3. Ferrule 1 illustrates a wafered ferrule which is formed by aligning and bonding the ends of two ferrule confined fibers and cutting one to the desired wafer length to give the wafer 20 bonded to the ferrule 1. The entire optical configuration is aligned within a fixture or holder, which maintains fiber alignment and allows the cavity length to be tuned without significant loss of alignment. For example, the holder can be provided with a piezoelectric transducer (PZT) actuator to allow the optical path length of the resonance cavity to be changed. This optical configuration provides for wavelength tuning and control with positioning accuracy of atomic dimensions.
FIG. 2 provides a schematic illustration of a conventional alignment and tuning fixture 40 for fiber FFP filters. Ferrules 1 and 3 (containing fibers 5 and 7, respectively) are held within ferrule holders 35 and 37 of fixture 40 with internal ends aligned and spaced apart to form an air gap. Ferrule holders 35 and 37 into which the ferrules are inserted and held in alignment are attached on opposing sides of a PZT element 36 which changes its length (along axis 25) upon application of a voltage. The PZT element 36 has an axial bore into which the ferrules extend and within which the FFP cavity is formed. Fixed frequency Fiber Fabry-Perot filters in which the cavity length is fixed at a selected length have also been described.
In contrast to lensed and microelectromechanical Fabry-Perot interferometers, FFP filters are generally robust and compatible with a wide range of field settings. To provide high throughput optical filtering, however, the optical fibers and ferrules comprising a FFP must be radially aligned to very high precision and accuracy and must be capable of maintaining good radial alignment during tuning. In addition, to achieve accurate optical filtering with minimized wavelength drift the optical fibers and ferrules comprising a FFP must be capable of maintaining a selected optical path length through the FFP resonance cavity. Accordingly, most ferrule fixtures and FFP assemblies include radial and longitudinal alignment systems to achieve and maintain good alignment. Examples of fixed and tunable FFP filters and holders for alignment and tuning are provided in U.S. Pat. Nos. 5,212,745; 5,212,746; 5,289,552; 5,375,181; 5,422,970; 5,509,093; 5,563,973; 6,241,397; and U.S. patent application Ser. No. 10/233,011, which are hereby incorporated by reference in their entireties to the extent that they are not inconsistent with the disclosure in this application.
Although conventional alignment fixtures have been shown to provide the precise and good alignment necessary for high optical performance, incorporation of these alignment systems in some applications has certain disadvantages. First, inclusion of elaborate alignment schemes often adds to the overall complexity of FFP filters, thereby, increasing the difficulty and cost of their fabrication. Second, many radial alignment systems are incapable maintaining good radial alignment while at the same time providing a selectably adjustable optical path length through the FFP resonance cavity. Therefore, these alignment systems are incompatible with tunable FFP filters. Other alignment schemes, while capable of providing tunable FFP filters, are susceptible to substantial deviations in alignment during adjustment of resonance cavity optical path length and, therefore, require periodic realignment. Finally, some radial and longitudinal alignment schemes are difficult, if not impossible, to effectively temperature compensate. Therefore, FFP filters employing these alignment schemes are susceptible to significant wavelength drift over a range of temperatures.
The present invention provides low loss FFP filters capable of maintaining good optical alignment and exhibiting high temperature stability temperature compensation. Particularly, the present invention provides a unitary fiber ferrule holder capable of achieving and maintaining the alignment of two optical fibers comprising a fixed frequency and tunable FFP. In addition, the present invention provides temperature compensated—FFP filters capable of maintaining good radial alignment and longitudinal alignment over a wide range of temperatures. Further, low cost instruments for mechanical sensing applications, such as monitoring temperature, pressure, and displacement, employing FFP filters with good radial and longitudinal alignment are presented.